Over the past few years a number of factors related to child care practices and environment (eg, sleeping position, thermal environment, and parental smoking) have been recognized as potentially important in the risk of sudden infant death syndrome (SIDS). Detailed studies of these factors have led to a greater understanding of normal development and of ways of possibly reducing the risk of SIDS. Campaigns to reduce the risk of SIDS have now been undertaken in several countries; in the United Kingdom, New Zealand, and Holland there have been marked falls in SIDS rates after such interventions. Recent physiologic studies of thermal balance, the role of infection, and the possibility of carbon dioxide rebreathing have shed considerable light on possible mechanisms by which these factors may exert their effects.